The present invention relates to an opto-electronic measuring arrangement having the features of the preamble of claim 1.
An opto-electronic measuring arrangement of such kind functions according to the HALIOS® principle developed by ELMOS AG with a (purely) optical transmitter basic coupling. This measuring principle is known in the related art and is described in various documents including U.S. Pat. No. 5,666,037; EP 0 706 648 B1; EP 1 671 160 B1; DE 100 01 955 A1.
The measuring arrangement, hereafter referred to as the “sensor”, comprises a compensation light source, hereafter referred to as the “compensator” as well as a transmission light source, hereafter referred to as the “transmitter”. Both light sources are alternately energized by a separate, dedicated current driver to transmit light (normally in the IR spectrum) in the transmission phase or compensation phase respectively. For this purpose, a clock generator controls the current drivers with clock signals that are phase-shifted by 180° with respect to one another. The frequency may be in the range from a few to several hundred kHz.
Via a photodiode, an optical receiver receives a portion of the light that is emitted by both light sources and converts it into current alternating signals. After separation of direct current and low frequency signal components (generally originating from ambient light) the current alternating signals are forwarded through a high-pass function (a capacitor for example) to a transimpedance amplifier (TIA), which converts these current signals into voltages. In turn, these voltages are then assigned alternatingly to the transmission phase and compensation phase in a synchronous demodulator and forwarded to a control unit. The control unit has the function to equalize the amplitude of these two signal components. To do this, the control unit adjusts the amplitudes of the currents through the compensator and the transmitter as necessary. Depending on the application, it is also possible to adjust only the amplitude of the compensator current with constant transmitter current amplitude or vice versa only the amplitude of the transmitter current with constant compensator current amplitude. The amplitudes of the compensator current are usually in the range of a very few mA. Depending on the application, the transmitter current amplitudes may be in the range from a few mA to several hundred mA.
The light emitted by the transmitter into the area surrounding the sensor impinges on the object that is to be measured (to be detected) outside of the sensor. This object reflects a fraction of the impinging light back to the photodiode of the sensor. The ratio between the current received by the photodiode and the transmission current used therefor is the coupling factor of the transmitter-measurement object-photodiode path, which the sensor determines and reflects in its controller correction signal.
Another (usually smaller) fraction of the transmission light reaches the photodiode within the sensor, and thus independently of the measurement object. This is the fraction of light that corresponds to the (internal) optical coupling factor of the transmitter-photodiode path (hereafter referred to as the optical basic coupling “OBC”). This is a (purely) optical coupling factor, since it depends entirely on geometric parameters (distances, angles) and material properties of the optical path (reflection or attenuation) within the sensor. In mathematical terms, the optical basic coupling is the ratio between the current generated by the transmission light in the photodiode without a measurement object and the transmission current used therefor. Accordingly, only the fraction of the transmission light that is transferred to the photodiode via the light path located in the sensor is considered.
The optical basic coupling is constant and in most cases is defined primarily by the sensor cover. For example, if an LED is used as the transmitter and an LED transmission current of 50 mA generates a photodiode current of 50 nA, the value of the optical basic coupling OBC in this case is 1:1000000, or expressed otherwise, 1×10−6.
The compensator is designed so that the emitted light is (practically) unable to reach the measurement object but instead is guided to the photodiode inside the sensor. In practical application, the light component emitted by the compensator is set to a predefined value, so that only a certain (usually small) fraction of the radiated light impinges on the photodiode of the sensor during the compensation phase. The optical coupling factor of the compensator photodiode path is the ratio between the current generated in the photodiode from this light component and the compensation current used therefor. The coupling factor is constant. Since light emitted by the compensator essentially does not reach the measurement object, it thus represents an immutable value or reference for the measurement. For example, if an LED is used as the compensator and the LED compensator current of 1 mA generates a photodiode current of 50 nA, the value of the compensator coupling in this case is 1:20000, or expressed otherwise, 50×10−6.
Since both the light originating from the transmitter and from the compensator travels along the entire receiver path including the photodiode, both signal components are influenced to the same degree by the transmission characteristics of the entire receiver path. The receiver path includes the high pass and the transimpedance amplifier as well as the photodiode. In mathematical terms, the transmission function of the entire receiver is canceled out in the system equations, and with it all relevant interfering dependencies such as the light sensitivity or temperature dependency of the photodiode. This explains, among other phenomena, the high degree of independence from extraneous light of the HALIOS® measuring method. The sensor remains functional even in full sunlight (100 klx). Even under these adverse ambient light conditions, it is entirely capable of reliably detecting an object within a defined measurement range. Movements of this object may also be detected. Thus for example it is possible to distinguish the approach of a hand or a wiping motion thereof in different directions. Touches of the sensor with a finger (tapping) may also be interpreted as switching functions. When equipped with a powerful optics system, a sensor that works according to the HALIOS® principle is able to detect an object, an item of luggage on a conveyor belt, for example, even at a considerable distance, for example 5 m or 10 m.
The optical transmitter basic coupling (OBC) determines the control unit resting value for the entire measuring arrangement. If no measurement object is in the vicinity of the sensor, no light is reflected outside of the sensor to the photodiode during the transmission phase. The sensor system is only adjusted to the optical basic coupling (OBC). It is decisive in influencing a whole range of properties of the sensor: for example, the optical basic coupling defines the sensor's sensitivity. If the optical basic coupling of the sensor is high, only a relatively small part of the light emitted by the transmitter is modified by the measurement object. Consequently, the sensitivity of the sensor is correspondingly low. This is equivalent to a small measuring range of the sensor. Conversely, a small optical basic coupling indicates high sensor sensitivity, which in turn enlarges the measurement range of the sensor. At the same time, a small optical basic coupling value also means that the resting value is more affected by noise, since the sensor system must function with little, or less, energy because the energy-rich signal component of the (missing) measurement object is entirely absent from this operating point. Noisy signals slow down the adjustment process, which in turn results in a slower response time of the sensor if the measurement object is moving away from the sensor.
The optical basic coupling is thus an important parameter in any sensor that works according to the HALIOS® principle, and it must therefore be tuned very precisely. With the measuring arrangements known from the related art, the settings for the optical basic coupling are made exclusively by means of structural features and changes to the sensor. The adjustment of the sensor is therefore very delicate and can only be made for a fixed value. Particularly with measuring arrangements (sensors) that include multiple transmitters positioned at different distances from the photodiode, a defined, dedicated optical basic coupling with the photodiode is needed for each transmitter. The opto-mechanical problem that must be solved in order to do this is usually very complex and makes production and particularly the design of the sensor very expensive.
Accordingly, based on the existing related art, an object of producing an improved opto-electronic measuring arrangement presents itself. Such an arrangement should particularly offer the capability of simpler, optimized setting of the optical basic coupling.